You mention burping which could be the biggest clue you have. Hypochlorhydria could be a problem but it's cause is important!
If your weight is down then poor digestion and poor food breakdown does point towards hypochlorhydria.
A test you can do is to prepare a glass of acid such as lemon juice. Warming it would be helpful. Then time as you do this. Take a big mouthful and wash it around your mouth and mix it with saliva and then swallow. Repeat and time how long it takes to burp. Expect around 45 seconds.
Increased Intestinal Permeability aka Leaky gut is linked to all of this. That is why you react to all foods. There is no easy way of stopping it except for consuming some fat before you eat which can lessen it. Some fish oil is a good choice.